As is well known in the art, a core of a plate and fin heat exchanger is typically fabricated of light-weight, braze clad aluminum-based metal sheets formed into layers that direct fluids in a heat exchange relationship. The formed layers are most often assembled into a stack and temporarily secured in fixed positions by a brazing fixture. The fixture is then placed within a vacuum furnace that creates a non-oxidizing atmosphere, and the temperature within the furnace is raised to melt braze alloys in the braze cladding, thereby permanently securing the layers into a core stack. It is critical that the temperature within the furnace be kept below the melting temperature of the aluminum-based metals making up the sheets.
After removal from the furnace, a variety of components may be secured to the stack such as fluid directing headers or manifolds, bosses for securing the stack to test apparatus, alignment flanges, etc., to complete fabrication of the core. Such components are secured to the stack through different methods including mechanical fastening, welding, and/or subsequent or secondary brazing. The core is then affixed to a variety of fluid directing conduits and related apparatus to allow it to operate in exchanging heat between a working fluid and a heat exchange fluid.
Modern heat exchangers, such as those used in commercial aircraft, and/or near zero-gravity environments, must operate under precise high pressure load tolerances, while meeting strict weight limitation requirements. Therefore, the quality of the bonds created by the braze alloys in the braze cladding must be very consistent. Accordingly, when heating the brazed stack for securing the components through further brazing, it is imperative that the temperature at any point in the stack not exceed the melting temperature of the previously melted braze alloys to avoid damage to the stack through localized weakening, etc. of the previously brazed layers.
Consequently, many efforts have been made to create low temperature braze alloys, such as those described in U.S. Pat. No. 4,040,822 to Stern, which Patent is incorporated herein by reference. Most such low temperature braze alloys are specifically directed to brazing non-ferrous metals such as aluminum, and the braze alloys typically include at least 0.1% to 5% magnesium, 0.1% to 5% rare earth elements, 4% to 15% silicon, with the balance being aluminum.
While those efforts have produced satisfactory low temperature braze alloys for limited working environments, the resulting braze alloys are restricted to a specific melting temperature, and therefore are appropriate only to brazing a specific base metal. Modern heat exchangers, however are increasingly designed for very demanding work environments and must exhibit ever decreasing total weight parameters. Consequently, new non-ferrous metal alloys are being used in thinner sheets, while the components added on after the first braze operation are likewise being fabricated of lightweight, low melting temperature metals. Additionally, such heat exchangers are susceptible of thermal fatigue stress upon repeated exposure to high temperature braze procedures. Finally, the increasing cost of replacing modern heat exchangers mandates a substantial effort at repair of core leaks, through further brazing treatments. While a low temperature braze alloy may be selected from the prior art that achieves a satisfactory braze for the variety of aforesaid braze requirements, a substantial number of braze alloys must be accessible, or stored and used to efficiently satisfy those requirements.
Accordingly, it is a general object of the present invention to provide an improved braze alloy that overcomes the deficiencies of the prior art braze alloys.
It is a more specific object to provide an improved braze alloy that affords a range of braze temperatures.
It is another particular object to provide an improved braze alloy that affords customizing the alloy to a specific melting temperature, to best satisfy unforeseen braze temperature demands.
It is yet another object to provide an improved braze alloy that minimizes cost and storage problems of prior art braze alloys.